Optical effects of Gemstones
“To suppose that the eye... could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.”
Charles Darwin (1809-1882), Origin of Species
Certain gemstones posses particular optical effects, referred to as ‘phenomena’ by gemologists, which are an intrinsic part of the precious stone’s structure. A good cut, in addition to adding value, can also contribute to highlighting these unique optical effects or, if necessary, concealing them. The Cat’s Eye optical effect, asterism and color change are only a few particularly desirable optical properties. Without these properties, our ancestors would never have considered gems so rare, magical and fascinating, and our current choice of precious stones would be far more limited.
- Adularescence (Opalescence)
- Color change
- Play of color
- Pleochroism and Double Refraction
Adularescence may not be the easiest word to remember, but it is very easy to spot. Adularescence is the bluish-white light that shimmers and glides over the surface of that enchantingly romantic gem Moonstone.
Adularescence is due to ‘interference phenomena’, which simply means the scattering of light by thin layers of albite crystal, more or less iso-oriented within the basic structure of the colorless monocrystal of the gemstone. This effect is named after a variety of Moonstone found in the Swiss Alps (mount Adula), called ‘Adularia’, and is referred to as ‘Opalescence’ when found in Opals.
Imagine being the first person to look into a gem and see a luminous star. No wonder star gems were once regarded with so much superstition. Asterism, also known as the star effect or asteria, is a uniquely beautiful, mysterious optical effect especially coveted in Rubies and Sapphires. Asterism is caused by reflections of light in two or more different directions. The phenomenon is due to long needle-shaped inclusions arranged symmetrically.
To obtain a star effect (with four, six and, rarely, twelve rays), the gemstone must be cut ‘en cabochon’ (a convex, highly polished form, with no facets). Asterism is most visible in a single, direct beam of light. Color aside, a well cut star gemstone has a distinct star whose rays are straight and equidistant Stars are typically positioned in the center of the gem, although they are sometimes deliberately off-centered for aesthetic reasons.
Another ‘scence’ word, this one originated in an 18th-century Venetian glassworks, when copper filings accidentally fell into a batch of molten glass.
According to legend, the workers exclaimed, “a ventura”, which means ‘by chance’, on noticing their glass with sparkles. In gemstones this phenomenon is caused by glittering metallic inclusions which cause a certain sparkle. In Chalcedony Quartz, thousands of tiny metallic flakes create the glimmering rarity called Aventurine and when present in Feldspar, sparkling Sunstone. One ‘gem’ to be aware of is Goldstone, a man-made glass and copper substitute.
Chatoyancy is an optical phenomenon similar to the typical Cat’s Eye, causing the gemstone to reflect light in an iridescent manner. Also known as the cat’s eye effect, it appears as a single bright, mobile, reflective line of light (similar to a cat’s eye), produced by the reflection of light from long needle-shaped inclusions occurring in a parallel arrangement.
Cat's Eye Silimanite
Similar to star gems, the cat’s eye effect is dependent on a gem being cut ‘en cabochon’ (in a convex shape) and is most visible in a direct, single beam of light. Another gemstone that displays this phenomenon is Chrysoberyl, so much so that if you say ‘Cat’s Eye’ to a gemstone professional, he or she will likely assume that you are referring to Cat's Eye Chrysoberyl.
Alexandrite and the phenomenon of color change
Physicist Manfred Eickhorst, well-known for his contributions to the development of gemological equipment and gemstone analysis, states, “The beauty of gems to the human eye is determined by the gemstone’s color appearance”. Obviously, this beauty is dramatically enhanced if a gem can change its color. Gemstones that display this phenomenon include the miraculous Alexandrite, which can appear blue green, forest green, green, khaki, teal or yellowish-green, and combinations thereof in natural light, and brownish-red, orangey-red, red or reddish-purple, and combinations thereof, in incandescent light (candlelight or electric lighting). This is also referred to as the ‘Alexandrite effect’. Color changing gemstones display different colors when exposed to different light sources. As revealed in a rainbow, white light is comprised of single colors of the spectrum: blue, green, orange, red, violet and yellow. When white light passes through a gem, some of these spectral colors are absorbed. The spectral colors of white light least absorbed combine to produce the color of the gem. It is changes to the component colors of a light source that cause gems to appear different under different lighting. Although they appear similar to our eyes, sunlight has very strong blue wavelengths, while electric light is richer in red wavelengths. In color change gems, absorption of different colors of the spectrum from different light sources results in our ‘color change’ perception. Other color change gems include Color change garnet, Color change Sapphire and Zultanite. A color change gem is rated by the strength of its change, and how attractive and distinct its color is in both candescent and incandescent light.
Iridescence, from the Greek ‘iris’, meaning ‘rainbow’, is the optical effect caused by a gem’s structure breaking up light into the colors of the rainbow. A very common Iridescence phenomenon , with colors that change depending on the angle of observation is seen in soap bubbles and butterfly wings.
The same ‘metallic Iridescence’ effect or ‘Schiller’, with a dominant electric blue, is aptly called ‘Labradorescence’ when present in Labradorite. The delicate Iridescence observed in pearls is called ‘orient’ or ‘overtone’. Other examples of Iridescent gemstones include Ammolite, Fire Agate and Mother of Pearl.
Remember the science classes where light was split into the colors of the rainbow using a prism? If you do, you have already understood ‘play of color’.
Unique to Opal “play of color” is the flashes of color that change with the angle of observation. An Opal whose ‘play of color’ flashes are in the rare reds is generally more valuable than Opals with just blue and green, but as with all gemstones, color preferences are subjective and should be dictated by individual tastes.
In certain gemstones, atoms are arranged so that light entering the crystal is refracted and at the same time split into two separate components or rays. This is called ‘double refraction’ or ‘birefringence’. Zircon is a good example of a doubly refractive gemstone. If these rays of light possess different colors visible when the gem is viewed from different angles, then this is called ‘pleochromism’ or, more technically, dichromism (two colors, such as Tourmaline) and trichromism (three colors, such as Tanzanite). A common misconception is that double refractivity makes a gem more brilliant. It does not. In fact, singly refractive gems can display colors that are purer and more intense than those seen in doubly refractive gems of the same color (e.g. Noble Red Spinel versus Ruby). Confused? The word that causes the problem is ‘double’.
A birefringent gemstone: Ratanakiri Zircon
While light is being split into two (doubled), the amount of light exiting the gem is not. While pleochroism can create unique colors in gemstones, sometimes adding richness, it affects gem buyers in three ways:
• the component colors are so similar that pleochroism usually doesn’t negatively affect beauty (e.g. Ruby and Sapphire);
• The component colors are different and not all are attractive, so cutters minimize pleochroism by orientating the crystal to display its single best color (e.g. Kunzite and Tanzanite);
• or the component colors are different, yet attractive, so the cutters orient the gem to get a pleasing mix of colors (e.g. Andalusite and Blue Green Tourmaline).
Andalusite, pleochromatic gemstone